In conventional telecommunication switching systems, more particularly telephone switching systems, analog signal transmissions, which are continuous in time, occur in transmission channels physically or spatially separated from one another. Modern telephone switching systems have tended not to utilized the space division principle, but the time-division multiplex principle, according to which analog signals, which are discontinuous in time, are transmitted. Of particular importance are the telephone switching systems in which a transmission of digital signals (which are also discontinuous in time) takes place. In this connection, the pulse code modulation (PCM) is used wherein, at periodically consecutive instants, the instantaneous values of the amplitudes of the voice signal are characterized through binary words, which are subsequently transmitted.
A basic task of a PCM time-division multiplex central office lies in through-connecting the binary words occurring on the time-division multiplex lines of the PCM receiver leading to the central office in time channels which on these lines are allotted to the individual connections, to the desired connection in accordance with selected time-division multiplex lines of the PCM sender leading away from the central office. Thus, in the course of a time-division multiplex procedure a time channel conversion must be carried out from the time channel seized on the incoming time-division multiplex lines to the time channel to be seized on the outgoing time-division multiplex line. Further, a spatial through-connection must be effected from an incoming time-division multiplex line to the selected outgoing time-division multiplex line.
In accordance with the above tasks in time sharing technology, an essential part of the switching systems are switching networks comprising time circuits and spatial circuits. The timing circuits have intermediate storages in which the PCM signals are stored intermediately during the time interval between incoming and outgoing time channel. The spatial circuits comprise crosspoint switches operated periodically and by pulses, over which incoming time-division multiplex lines are capable of being connected with outgoing time-division multiplex lines. Depending on the functional sequence of the timing circuits and spatial circuits, several structures of switching networks are possible. The choice of circuit used depends on the conditions imposed on the switching system.
For example, the space-time-space system (S-T-S) comprises a spatial circuit at the input end, to which are connected the storages of the timing circuit, and the outputs of which are connected with the inputs of a second spatial circuit at the output end. This structure of the switching network has the characteristic that the centrally arranged intermediate storages of the timing circuit can be utilized repeatedly in time sharing, and it is particularly of advantage in synchronous telecommunications networks, since they need no storages at the input and output of the switching centers. A disadvantage of this switching network structure is the fact that the expansion possibility of switching centers so constructed cannot be carried out in small steps with economically justifiable expenditure, since a minimum number of storages must be committed in advance with the basic expansion. Compared thereto, switching networks with a time-space (T-S) structure are more advantageous. At the input end, they have timing circuits allocated to individual lines for the conversion from incoming time channels to outgoing time channels, to which are connected the switching networks having crosspoint contacts which are operated during the time channels to which conversion has taken place through the timing circuit. Switching networks, so constructed, can be expanded without difficulty and are of interest especially for asynchronous switching systems in which intermediate storages are at any rate needed at the input end for purposes of synchronization.
The transmission of PCM signals in the course of such switching can be accomplished either according to the serial bit method, i.e., the individual bits of the PCM signals are transmitted sequentially, or according to the bit parallel method, i.e., all the bits of a PCM signal are transmitted simultaneously. Parallel bit transmission requires either a serial to parallel or parallel to serial converter at the input and output end and, at least at the input end, a storage is required in which the PCM signals for the duration of a pulse frame can be stored intermediately. Furthermore, an increased expenditure for crosspoint switches is required in accordance with the number of bits per PCM signal. However, since in the case of parallel through-connection only a fraction of the through-connection time is required, as is required for series through-connection, the crosspoints and collecting mains disposed within the spatial switching network can be considerably better utilized so that the parallel through-connecting in comparatively large storage systems leads to great economy.
In a time-division multiplex switching network of known construction (West German Examined Application No. 1,762,270) using the T-S structure, and wherein the transmission of the PCM signal takes place according to the parallel bit method, the expansion possibilities mentioned hereinabove, which are virtually unlimited by the system, are, likewise, provided in principle. Even if this type of time-division multiplex switching networks can readily be adapted to the particular telephone traffic to be controlled through corresponding expansions, certain limits remain, because very large spatial switching networks no longer offer an economically acceptable solution, due to the wiring difficulty of them.
It is an object of the invention to provide a time-division multiplex switching network using the T-S structure, wherein expansions are readily feasible and wherein, even if very large switching networks arise as a result, the amount of apparatus required, particularly for the spatial switching network therein, is held within reasonable bounds.